There is more smog and soot along the shoreline than elsewhere in the state — a fact of life in Connecticut for years, confirmed once again in a yearly report released Wednesday by the American Lung Association.

Fairfield and New Haven counties had the worst scores for both ozone, or smog, and particle pollution, or soot, in Connecticut, the report says. Those two counties received failing grades for ozone levels, as did Middlesex and New London counties.

The shoreline suffers from the high-traffic I-95 corridor and downwind pollution from coal-fired power plants in the Midwest, said Michelle Marichal, acting director of health education and public policy for the Connecticut office of the lung association. Large seagoing vessels are also thought to play a role, she said.

Despite the failing grades in some counties, the long-term trend in Connecticut is positive, Marichal said. "Even though we failed in a lot of the counties, we are seeing improvements in high-ozone days," she said.

For example, she said, in 1996 — when the lung association's first 'State of the Air" report came out — Hartford County had nine "high-ozone" days, and in Wednesday's report it had two.

Also, every participating county in Connecticut received a passing grade for soot, though data was not collected Middlesex, Tolland and Windham counties.

The Connecticut county with the best air overall was Litchfield.

This year's report uses air pollution data compiled by the U.S. Environmental Protection Agency over three years, from 2009 to 2011. This is the group's 14th annual report.

In the three-year period, Fairfield County had 32 "orange" days, when the smog level was unhealthy for sensitive groups — more than twice as many "orange" days as any other county in Connecticut. It had two "red" days, when the smog level was unhealthy for the general population. Litchfield County, on the other hand, had three "orange" days and no "red" days.

Nationally, California had some of the worst scores, with Bakersfield-Delano ranking No. 1 for soot and No. 3 for smog. Los Angeles-Long Beach-Riverside topped the national ranking for worst smog.